Hi Nocturn,
I used to keep betta's with other fish, in large tanks (29 and 75 gallons). I no longer do so. Here's the why's..
First, bettas do not eat as much as other fish, and shouldn't. If you add other fish, you have to feed them. The betta will eat their food just as readily as they will, and won't know its bad for him. He will eventually get really bloated, and will die from it. I lost my last betta to this.
Second, despite their famed aggression against other male bettas, they aren't much for defending themselves. Other fish are quicker and tougher, and in such a small environment he might not fare well.
Next, on to your snails. Black Mysteries are great snails, I keep quite a few. They keep my tanks quite clean of algae. Don't worry if he's hiding in his shell. He can do this for days at a time, then be normal. Do make sure he can't climb out though. They will. They aren't trying to escape or anything, they just wander around. They can survive for quite a while out of the water so if it does happen and you catch it within 24 hours, just drop him back in. He'll probably have his shell tightly closed and just sit there for a while.. then back to normal.
The main suggestion I've seen for tanks of this size and bettas have been Dwarf Gouramis. Be careful though. The dwarves are known to be somewhat aggressive, and they're kinda 'betta' cousins. They share a lot of characterists. They come in some great colors though, and you should be able to work something out. You might want to partition the tank initially to see if they try to fight through it.
Oh, and just one of them